This invention relates to the switching of optical power in an active branching waveguide.
The operation of passive branching waveguides has been described in an article by W. K. Burns and A. F. Milton, IEEE J. Quantum Electron, QE-11, 32, 1975 and by H. Yajima, Applied Physics Letter, 22, 647 (1973). These articles show that if the branch taper is sufficiently slow, modes will propagate through the branch adiabatically. Adiabatic propagation implies a slow enough change in waveguide parameters that the power in the lowest-order, local normal mode remains in the lowest-order mode, and power in the second-order, local normal mode remains in that mode through the transition represented by the branch. There is no mode conversion, or power transfer, between the local normal modes. Also for sufficient separation of the branch arms, the local normal modes tend to change their distribution of optical power between the arms of the branch until they exist primarily in one arm of the branch or the other. For example, in a branch that supports two local normal modes, the lowest-order mode will select one arm of the branch and the second-order mode will select the other arm of the branch. An exception to this occurs if the branch is exactly symmetrical, then the modal evolution described above does not occur and instead the mode power associated with both local normal modes divides equally between the arms of the branch. Prior art U.S. Pat. No. 3,883,220 (Taylor) describes an active branching waveguide in which the index of refraction of one arm of the branch is reduced to or below the index of refraction of the substrate. That arm of the branch is said to be "cut-off" and propagation can only occur in the other arm of the branch. Prior art U.S. Pat. No. 3,795,433 (Channin) is similiar in that one or the other arms of the branch is electro-optically created, the other being cut-off.